Numerous implantable medical devices (IMDs) are available for acute or chronic implantation within patients. Some implantable medical devices may be used to monitor physiological signals of the patient, such as cardiac pacemakers, implantable hemodynamic monitors, implantable cardiac monitors (sometimes referred to as implantable loop recorders or ECG monitors), implantable blood chemistry monitors, implantable pressure monitors, etc. Among the various types of physiological sensors utilized by medical devices for monitoring patients are electrodes for measuring electrical signals and/or impedances, piezoelectric crystals, accelerometers, pressure sensors, pH sensors, acoustical sensors, temperature sensors, and oxygen sensors.
The physiological signals may be stored, processed and analyzed by the medical device to generate physiological data about a patient useful to a clinician in diagnosing a condition or planning medical treatment. Some implantable devices may be configured to deliver a therapy in conjunction with monitoring of physiological signals. Physiological signals may be processed and analyzed to determine when a therapy is needed or how a therapy needs to be adjusted to benefit the patient. Therapies delivered by an implantable medical device can include electrical stimulation therapies, e.g., cardiac pacing, cardioversion/defibrillation shock pulses, or neurostimulation, and pharmacological or biological fluid delivery therapies.
In order to provide reliable physiological data needed for determining a medical risk, detecting pathological conditions, controlling automatic therapy delivery or generally producing data in a form useful to a clinician for diagnosis and prognosis, reliable sensor signals are required. For example, patient activity level may be determined from an accelerometer in order provide rate responsive pacing at a heart rate that meets the metabolic demand of the patient. An accelerometer signal may be subject to noise or motion not directly associated with the motion of the patient activity or exercise, such as cardiac motion or respiratory motion. Methods are needed for identifying a sensor signal that provides acceptable signal to noise ratio for reliable signal processing.